This invention relates to an electron gun, particularly an in-line type electron gun, for use in a color cathode ray tube, and more particularly to an electrode structure that serves as a main lens of the in-line type electron gun.
One of the causes that influence the focus characteristic of a color cathode ray tube, e.g. a color picture tube is the diameter of the main lens of an electron gun in the tube. Desired focus characteristic can be obtained by using the main lens of as large diameter as possible.
However, it is very difficult to increase the main lens diameter in the in-line type electron gun. The reason is as follows. In the in-line type electron gun, three electron guns, corresponding to three colors of green, blue and red, arranged on a horizontal plane are integrated. Thus, if these three electron guns are incorporated in a neck tube with a limited diameter in the color cathode ray tube, the diameter of the cylinder that serves as the main lens of each electron gun and the interval between the main lens will be greatly restrained.
The above problem will be explained in more detail with reference to the drawings.
FIG. 1 is a sectional view of the color picture tube equipped with an in-line type electron gun of a conventional structure. FIG. 2 shows the detail of each grid electrode part in FIG. 1. In FIGS. 1 and 2, a phospher screen 3 with three color phosphors alternately applied thereon in the stripe shape is supported on the inner wall of a face plate 2 of a glass envelope 1. Central axes 15, 16 and 17 of cathode 6, 7 and 8 correspond to the central axes of their corresponding apertures of a first grid electrode 9 (referred to as G.sub.1), a second grid electrode 10 (referred to as G.sub.2), a third grid 11 (referred to as G.sub.3) that is one electrode of an electrode pair constituting a main lens and a shield cup 13, and are arranged substantially parallel to each other on a common plane (The direction along this common plane is hereinafter referred to as a horizontal direction). A fourth grid electrode 12 (referred to as G.sub.4 electrode) is the other electrode constituting the main lens. The central axis of a central aperture of the fourth grid electrode 12 corresponds to the central axis 16. On the other hand, as seen from FIG. 2, the center axes 18 and 19 of outer apertures of G.sub.4 electrode do not correspond to the corresponding central axes 15 and 17 but are slightly outwardly deviated therefrom. More specifically, the diameter L.sub.1 of the outer apertures of G.sub.3 electrode 11 is smaller than the diameter L.sub.2 of the outer apertures of G.sub.4 electrode 12 (L.sub.1 &lt; L.sub.2). Three electron beams emitted from the cathodes 6 to 8 are incident to the main lens consisting of G.sub.3 electrode 11 and G.sub.4 electrode 12 along the central axes 15, 16 and 17, respectively. Generally, G.sub.1 electrode 9 is biased to 0 V and G.sub.2 electrode 10 is 600 V to 800 V. G.sub.3 electrode 11 is biased to 7 kV to 10 kV lower than G.sub.4 electrode 12. G.sub.4 electrode 12 is biased to the voltage as high as 25 kV to 30 kV. Shield cup 13 and the conductive film 5 provided on the inner wall of glass envelope 1 are also biased to the same high voltage. The central apertures of G.sub.3 electrode 11 and G.sub.4 electrode are coaxial so that the main lens is in axial symmetry at the central part. Thus, after the central beam is converged by the main lens, it travels straight on the line along the axis 16. On the other hand, the outside apertures of grid electrodes 11 and 12, as seen from FIG. 2, have their axes deviated from each other so that the main lens is in non-axis-symmetry at the outside portion. Thus, the side beams corresponding to R (red) and B (blue) arranged at the outside portions pass the portion deviated from the lens central axis toward the center beam in the divergence lens region formed on the G.sub.4 electrode 12 side of the main lens region and are subjected to the convergence operation and concentration power toward the central beam direction by the main lens. Thus, the three electron beams are imaged and also converged so as to overlap each other on a shadow mask 4. The operation of converging the electron beams in this way is called static convergence. Each of the electron beams is subjected to color selection by shadow mask 4 and only its component exciting the phosphor of the color corresponding to each beam reaches the phosphor screen 3 through the opening of shadow mask. Further, on outer magnetic deflection yoke 14 is provided to scan the electron beams on the phosphor screen 3.
The causes that greatly influence the focus characteristic of the color picture tube mentioned above are a lens magnifying power of the main lens and an aberration thereof which are dependent upon the lens convergence operation. Generally, weakening the lens convergence operation of the main lens decreases the lens magnifying power and the spherical aberration, thereby improving the focus characteristic. One of the methods of weakening the lens convergence operation is to enlarge the apertures of G.sub.3 and G.sub.4 electrodes that constitute the main lens.
However, in the in-line type electron gun as shown in FIG. 1, the main lenses corresponding to three colors of R, G and B are arranged on the same horizontal plane so that the diameter of the above apertures must be 1/3 or less of that of the neck tube incorporating the electron guns in glass envelope 1. Further considering the problem on electrode manufacturing, the allowable critical value thereof will be smaller.
The inventors of this invention proposed a method for effectively increasing the above critical value of the apertures in JP-A-59-21564. The method disclosed therein is to increase the effective aperture diameter of the lens of converging the outer portion of side beams as large as possible, thereby removing halo in the side beams to improve the resolution of the color picture tube.
FIG. 3 shows a perspective view, partially broken, of the electrode structure of the main lens portion disclosed in the above JP-A-59-215640 and FIG. 4 is a front view of the G.sub.4 electrode 12 side viewed from the G.sub.3 electrode 11 side in the direction of line IV--IV. As shown in FIG. 3, inner electrodes 112 and 122, which constitute the facing plane of G.sub.3 electrode 11 and G.sub.4 electrode 12, are retreated therefrom by m.sub.1 and m.sub.2, respectively. Thus, the inner electrodes invade deep in the G.sub.3 and G.sub.4 electrodes, thereby providing the same effect as the increase of the aperture part thickness. Namely, the effective thickness of the main lens will be increased. The electrode structure shown in FIG. 3 is assembled, for example, by inserting the components into a mandrel or the like so that the center axis of elliptical apertures 123 and 124 of inner electrode 122 coincide with the center axis of the inner diameter of G.sub.4 electrode 12, and thereafter fixing them at positions 200 to 203 by laser welding. Incidentally, A.sub.1, A.sub.2, A.sub.3 and A.sub.4 denote a central axis, respectively.
From the viewpoint of parts manufacturing, it is desirable to make the outer diameter d of the inner electrode 122 equal to the inner diameter of G.sub.4 electrode 12. However, actually, the inner diameter D of G.sub.4 electrode 12 can be larger than the outer diameter d of inner electrode 122 (D &gt; d). In this case, if G.sub.4 electrode 12 is fixed by laser welding or the like, as shown in FIG. 5, G.sub.4 electrode 12 will be partially deformed and the end surface 12a of G.sub.4 electrode 12 will be also deformed. In FIG. 5, A.sub.4 and A.sub.5 denote the central axis of G.sub.4 electrode 12 before and after the deformation. The central axis will be deviated by a. Of course, if the outer diameter d of inner electrode 122 is larger than the inner diameter D of G.sub.4 electrode (d &gt; D), inner electrode 122 cannot be inserted into G.sub.4 electrode 12, thus making the assembling impossible.
The main lens is formed between the end surface 12a of G.sub.4 electrode 12 and the elliptical apertures of inner electrode 122 and between the end surface of G.sub.3 electrode and inner electrode 112. If the main lens is formed in the state where the end surface 12a of G.sub.4 electrode 12 is deformed, the electric field thus formed will be confused, thereby distorting the main lens. Thus, the beam having passed the main lens will have astigmatism to deteriorate the resolution.